Stray electrical currents associated with farm installations, particularly dairying equipment, can present a significant economic problem for farm operations. Dairy operations are susceptible to stray electricity because cows are extremely sensitive to electricity, much more so than humans, and will respond to potentials as low as one volt or less. Such problems are described in a report by H. A. Cloud et al. "Stray Voltage Problems with Dairy Cows" North Central Regional Extension Publication 125.
An advantageous solution to an aspect of the problem involves opening the link between the primary and secondary neutrals of the transformer serving the farm. However, this link must be closed very rapidly any time the voltage between the neutrals exceeds a predetermined level (i.e., as might be caused by a transformer failure, lightning surge, or other surge condition). In U.S. Pat. No. 4,958,250, issued to Kotski on Sep. 18, 1990, an isolator/surge protector is disclosed to mitigate the cause of stray voltage problems. An electronic switch circuit is disclosed which will close on 60 hertz (Hz) overvoltage conditions or ion lightning surges of several tens of thousands of amperes having rise times in the order of 1 to 10 microseconds. The high speed electronic switching apparatus which is connected between the primary and secondary neutrals of the distribution transformer normally provides a very high impedance between the primary and secondary neutrals to both AC and DC. These types of isolator/surge protectors are sometimes used with metallic systems which are cathodically protected by an external DC bias which prevents corrosion from being initiated. Unfortunately, this external DC bias can operate to hold the electronic switches of a surge protector in a conductive state once triggered by a surge event or transient event because the external DC bias may be greater than the turn OFF voltage of the switch. The external DC bias often holds the switch ON even though the event which caused the triggering condition has ended. In this condition, the switch is "stuck" in an ON state. Thus, the external DC bias can prevent proper operation of the isolator surge protector.
Isolator/surge protectors may also be used in systems which protect metallic structures against corrosion so that the metallic structures operate safely and experience failures less frequently. Many metallic structures and systems must be protected against corrosion. For example, metallic gas transmission and distribution lines must be protected against corrosion to prevent gas leaks, particularly in certain environments. Further, metal encased high-voltage underground transmission lines should be corrosion protected. Underground transmission lines commonly consist of three paper insulated conductors encased in a single metal pipe which is filled with oil and pressurized. Any small pinhole in the pipe due to corrosion can cause a cable failure (i.e., line-to-ground fault) causing considerable economic loss and customer complaints. Similar situations exist with many other metallic objects which can cause economic or safety concerns when allowed to corrode.
The most common method of corrosion protection of metallic systems is to make the system to be protected more negative in potential than any other metallic object with which it is in electrical contact. A common method to accomplish this is to insulate the object that is to be corrosion protected (e.g., such as by applying an insulating coating), and to isolate it from other objects. A negative DC potential is then applied to the system relative to ground, with typical values being in the 0.6 volt to 3.0 volt range. While this procedure may eliminate corrosion, it introduces a second problem if the corrosion protected system is an inherent part of a 60 Hz power system (or 50 Hz in European countries) or if it is coupled to such a power system through resistive, capacitive, or inductive coupling. In the event of a fault (e.g., a short circuit) within the power system, the electrically isolated, corrosion protected system may rise in voltage to unsafe levels, which is not acceptable. To prevent such corrosion protected systems from reaching unsafe voltage levels in the event of a fault, lightning, switching transient, or other system disturbance, it would be highly desirable if the corrosion protected system were connected to ground through a device that would present a high-impedance to DC, at least up to the DC voltage level of interest (which may be up to 10 volts when stray DC influences are considered), but presents a low impedance to AC at all times so that the voltage of the corrosion protected system is limited to values safe for personnel and equipment.
To date, such isolator/surge protector functions have been performed by a device known as a polarization cell, an electromechanical device which has the ability to present a relatively high impedance to DC (up to about 1.2 volts DC) and simultaneously present a low impedance to AC. Among the several problems with the polarization cells are that it is often necessary to connect several in series to isolate to the desired DC voltage level, it is an electromechanical device which requires routine maintenance and eventual disposal of the electrolyte, and the electrolyte is extremely caustic and hazardous.
An isolator surge protector (ISP) may also be used to isolate DC current and transmit AC current for power transformers which are not designed to accommodate a DC current flowing through the transformer windings. DC currents as low as several amperes can cause partial core saturation, resulting in excessive reactive power losses in the transformer (i.e., excessive heating), a drop in system voltage, the introduction of undesirable harmonics, and a significant increase in noise level. Sources of DC current that can cause this problem include geomagnetically induced currents caused by solar flares, stray DC current from rapid transit systems typically found in large cities, and stray DC current associated with high-voltage DC transmission systems particularly when operating in the monopolar mode (i.e., earth return mode). In such applications, it may be necessary to block up to 4,000 volts DC while simultaneously carrying up to 200 amperes AC, with the ability of the isolating device to carry power system fault currents up to 60,000 amperes and withstand lightning/switching transients, all while preventing hazardous voltages from being developed across the two points to which the ISP is connected. In other applications, an ISP may be used to prevent unsafe voltages between parts of corrosion protected systems (e.g., such as across an isolated flange in a gas pipeline).